JP3248438B2 - Electrolytic chromate treated steel sheet for welding with excellent alkali content properties and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic chromate-treated steel sheet suitable for use as a material for 18L cans, pail cans, art cans and the like and capable of performing non-polishing welding.

[0002]

2. Description of the Related Art Electrolytic chromate-treated steel sheets having a chromate film composed of a metal chromium layer and a chromium oxide layer on the surface of the steel sheet are used as materials for beverage cans, food cans, art cans, 18L cans, pail cans and the like. In recent years, its application range tends to be more and more widespread because it is inexpensive and has excellent paint adhesion compared to tinplate. On the other hand, the disadvantage of electrolytic chromate-treated steel sheet compared to tinplate is the lack of weldability.Currently, the chromate film on the surface layer that inhibits weldability is mechanically polished and removed immediately before welding before welding. ing. However, when such a method of mechanically removing the chromate film is performed in industrial mass production, a large amount of polishing dust (metal powder) is generated, and the polishing dust is mixed as foreign matter into the can. This causes problems such as an increase in the load required for maintenance such as cleaning of the welding machine, and a risk of fire occurring due to accumulated metal powder. Furthermore, in such polishing welding, there is also a problem that the exposed metal surface is oxidized due to heat input during welding and changes to dark blue or black, and the appearance is deteriorated after welding.

For this reason, the following proposals have been made on the premise of non-polishing welding to improve the weldability of electrolytic chromate-treated steel sheets. (1) After a steel sheet is subjected to electrolytic chromate treatment, the steel sheet is immersed in a high-concentration chromic acid bath to dissolve and remove the surface chromium oxide layer, thereby forming a metal chromium layer having an adhesion amount of 45 to 90 mg / m ^{2 on} the steel sheet surface. A method of forming a chromium oxide layer having an adhesion amount reduced to 1 to 10 mg / m ² , thereby lowering the contact resistance of the chromate-treated film to secure weldability (Japanese Patent Publication No. 6-3 / 1994)
7714 No.) (2) electrolysis amount adhering to the steel sheet surface adhesion amount a smooth metallic chromium layer of 40~200mg / m ² is excellent in weldability and corrosion resistance by forming a chromium oxide layer of 4~25mg / m ² (3) Electrolytic chromate treated steel sheet in which at least one side of the steel sheet has a granular or angular metallic chromium projection (Japanese Patent Publication No. 7-26236) (4) Electrolysis Steel plate combining chromate treatment and trace tin plating (Japanese Patent Publication No. 7-94719)

[0004]

However, these prior arts have the following problems. First, regarding prior art (1), 18L cans and pail cans using electrolytic chromate-treated steel sheets are often filled with an alkaline solution containing a surfactant as a content, and the corrosion resistance in such an alkaline solution is high. Is mainly affected by the amount of deposited metal chromium layer and the form of chromium metal.
A chromium metal layer having an adhesion amount of about g / m ² causes a problem in terms of corrosion resistance in a certain alkaline solution. In addition, since this prior art (1) re-dissolves the chromium oxide layer once formed, local unevenness in the thickness of the chromium oxide layer inevitably occurs due to the difference in dissolution rate, and Fluctuations in surface resistance (contact resistance) are inevitable. Such a change in contact resistance makes the heat generated during welding unstable, so that the range of current that can be welded (hereinafter referred to as ACR )
Is reduced.

Further, in the steel sheet shown in the prior art (2), since the amount of the chromium oxide layer adhered is too large, it takes time to break the film by the electrode at the time of welding, and the fluctuation of the heat generation state with respect to the current fluctuation is large. . For this reason, welding can be performed at a wire speed of 5 m / min or less, but it is difficult to secure a stable energized state by high-speed welding at a wire speed of 15 m / min or more, which is applied in the manufacture of 18L cans and pail cans. There is a problem that it cannot be secured.

The electrolytic chromate-treated steel sheet shown in the prior art (3) exhibits a reddish-brown color due to the absorption of short wavelength light by the unevenness of the metal chromium layer. For this reason, the film shown in the prior art (3) cannot be applied to both sides,
Further, even if applied to only one side, there is a problem that complicated front and back management (identification management of the front and back sides when the printing target side needs to be the front side of the steel plate) must be performed.
In addition, the contact resistance of the plate / plate interface during welding (the interface between the steel plates at the overlapped portion of the steel plates during welding) is reduced by adopting the structure of the metal chromium layer as shown in the prior art (3). The contact resistance at the interface (electrode / plate interface) between the soft electrode (copper wire) and the steel plate used for welding the can (electrode / plate interface) does not decrease because the chromium oxide layer is difficult to break. Therefore, there is a problem that excessive heat is easily generated at the electrode / plate interface before the plate / plate interface properly generates heat, and stable welding is difficult.

[0007] The steel sheet shown in the prior art (4) has good weldability since the coating contains tin having a low melting point and good extensibility, but the tin dissolves in an alkaline solution, so that the steel sheet has a high melting point.
There is a drawback that it cannot be used for L cans, pail cans and the like.
Accordingly, an object of the present invention is to provide a good surface color tone, no need for front / back management, excellent bare corrosion resistance in an alkaline solution containing a surfactant, and excellent non-polishing welding of 15 m / min or more. It is an object of the present invention to provide an electrolytic chromate treated steel sheet for welding having improved weldability and a method for producing the same.

[0008]

The features of the present invention for solving such a problem are as follows. (1) The surface roughness Ra is 0.15 to 0.50 μm and 1
The number of protrusions exceeding 25 μinch per inch is 10 to 12
A metal chromium layer having an adhesion amount of more than 90 mg / m ² per side and 150 mg / m ² or less on both sides of a steel sheet having a range of 0 PPI, and an upper layer having an adhesion amount of 3 mg / m ^{2 in} terms of metal chromium per side.
An electrolytic chromate-treated steel sheet having a chromium oxide layer of about 9 mg / m ² , wherein the steel sheet has a maximum diameter of 10 nm when present on the surface of the metal chromium layer and observed from above.
The number of metal chromium protrusions as described above is 6 × 10 ¹² / m
Electrolytic chromate treated steel sheet for welding excellent in alkali content resistance and weldability characterized by being less than ² .

(2) The method for producing electrolytically chromated steel sheet for welding according to claim 1, wherein the surface roughness Ra is 0.1.
After degreasing and pickling both surfaces of a steel plate having a thickness of 5 to 0.50 μm and having a number of protrusions exceeding 25 μinch per inch in a range of 10 to 120 PPI, 1/500 of the chromic acid concentration is obtained. In a chromic acid bath containing 1/50 concentration of sulfate ion, a current density of 70 ASD or more
In addition to performing the three-pass cathodic electrolysis, in the cathodic electrolysis, the cathodic electrolysis current is always supplied at a current density of 3 to 7 ASD even in a portion where the steel sheet in the chromic acid bath does not face the electrode. A method for producing electrolytic chromate-treated steel sheets for welding, which is characterized by excellent alkali content resistance and weldability.

Here, the above PP representing the surface roughness of the steel sheet
With I, an extraction curve of roughness corresponding to the surface roughness of the steel sheet is obtained, an average line of the extraction curve is obtained, and a reference height of 25 μinch is set as a reference level in the positive direction from the average line. When the extraction curve goes down to the negative side from the average line and exceeds the reference height (25 μinch) on the positive side, it is counted as one peak (one convex portion) and represents the number of peaks per inch. It is a thing.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the knowledge and facts on which the present invention is based, and the details and reasons for limitation of the present invention will be described. First, the result of an examination from the viewpoint of weldability will be described. As a method for improving the weldability of electrolytic chromate-treated steel sheets used for cans, the conventional technology refers to the properties of tinplate and reduces only the contact resistance at the plate / plate interface or the contact resistance between the electrodes / electrodes. Has been planned. However, as a result of investigations and studies by the present inventors, the weldability of electrolytic chromate-treated steel sheets in a high-speed wire seam welding machine used for welding of can materials is not necessarily the plate / plate interface or the electrode / electrode contact resistance. It was found that lowering the value did not improve it.

That is, in the welding of general cans, the conduction path at the electrode / plate interface (the contact width of the electrode) with respect to the conduction path of the electrode / plate interface (the interface between the steel plate and the electrode at the time of welding) is compared with the welding of beverage cans and the like. It has been found that, due to the large (overlap width of the steel plate), depending on the characteristics of the material, a peculiar phenomenon that cannot be seen in welding of beverage cans occurs. When welding a general can using an electrolytic chromate-treated steel sheet as a material, in addition to the contact resistance at the plate / plate interface, the balance of the contact resistance between the electrode (copper wire) / plate interface and the plate / plate interface is important for weldability. I found it to be. Specifically, when the contact resistance at the plate / plate interface is lower than that at the electrode / plate interface, excess heat is generated at the electrode / plate interface when the necessary current is applied to obtain appropriate heat generation at the plate / plate interface. This causes defects such as dust and splash on the outer surface of the welded portion. Therefore, in order to perform stable welding, it is necessary that at least the contact resistance between the electrode / plate interface and the plate / plate interface is balanced.

Further, when the contact resistance is large, the variation in the amount of generated heat with respect to the variation in the current density during energization becomes large, and the ACR becomes small. Therefore, in order to weld a steel sheet having a large contact resistance, it is necessary to suppress a change in current density during welding. Specifically, it is necessary to control fluctuations in welding conditions such as current control during welding, can feed speed, overlap width of a steel plate welded portion, and vibration of a welding machine. These fluctuations in welding conditions become unavoidable factors that impair weldability as the welding speed increases. As a result of the study by the present inventors, in order to reduce the contact resistance at the plate / plate interface and the electrode / plate interface in a well-balanced manner and to improve the weldability, it is necessary to reduce the chromium oxide layer of the electrolytic chromate film, It was found that control of the form of the chromium layer and control of the surface roughness of the steel sheet were essential.

First, in order to reduce the contact resistance between the plate / plate interface and the electrode / plate interface, a material steel plate having a surface roughness of a predetermined level or more is used and the amount of the chromium oxide layer deposited is not more than a predetermined level. Specifically, the surface roughness Ra is 0.15 μm or more, and 2 / inch
It is necessary to use a material steel plate having a number of protrusions exceeding 5 μinch of 10 PPI or more and to make the amount of chromium oxide layer attached (the amount of adhesion in terms of metal chromium, the same applies hereinafter) 9 mg / m ² or less. understood. By using a steel sheet having a surface roughness equal to or higher than a predetermined level as a material steel sheet and making the amount of chromium oxide layer adhered to 9 mg / m ² or less, for example, regarding the contact resistance value at the electrode / plate interface, 1/10 to 1/10 as compared with a normal electrolytic chromate-treated steel sheet having an adhesion amount of the chromium oxide layer of about 20 mg / m ^2.
It drops to about 0. However, this value is about 100 times larger than tinplate. The reason why the contact resistance is effectively reduced in this way is that the larger the surface roughness of the material steel sheet, the more easily the chromium oxide layer is broken during welding, so that the adhesion amount of the chromium oxide layer is reduced and the steel sheet surface is reduced. It is considered that this is because the chromium oxide layer is effectively destroyed by the electrode (copper wire) by giving the surface roughness.

On the other hand, it has been found that the contact resistance at the plate / plate interface of the electrolytic chromate-treated steel sheet largely depends not only on the amount of the chromium oxide layer deposited but also on the form of the metal chromium layer. A steel sheet having a surface roughness equal to or higher than the above-described predetermined level is used as a material steel sheet, and an adhesion amount of a chromium oxide layer is 9 mg / m ² or less, and an electrolytic chromate-treated steel sheet having various forms of a metal chromium layer, Those plates / plate interfaces and electrodes /
When the contact resistance at the plate interface was examined, the contact resistance between the plate / plate interface and the electrode / plate interface was almost the same for the electrolytic chromated steel sheet having a smooth metallic chromium layer. On the other hand, in an electrolytic chromate treated steel sheet in which relatively large diameter metal chromium projections are formed at a high density on at least one side of the metal chromium layer at the plate / plate interface, the contact resistance at the plate / plate interface is lower than that of the electrode / plate. 1/10 to 1/100 of the contact resistance at the plate interface
It was found to be reduced to the extent. This is presumably because the hard metal chromium projections are pressed against each other to destroy the upper chromium oxide layer. The contact resistance at the electrode / plate interface of the electrolytic chromated steel sheet was not affected by the form of the metal chromium layer.

An electrolytic chromate-treated steel sheet having a relatively large-diameter metallic chromium protrusion formed on the metal chromium layer on one side of the steel sheet at a high density, and an electrolytic chromate-treated steel sheet having a plate-like metallic chromium layer formed on both sides of the steel sheet. Each was continuously made at a welding speed of 15 m / min or more by using a welding machine for 18 L cans, and the ACR at that time was examined. As a result, the range of ACR was as follows. [Tinning]> [Electrochromate treated steel sheet with plate-like metal chromium layers formed on both sides of steel sheet]> [Electrochromate treatment where relatively large protrusions are formed at a high density on the metal chromium layer on one side of the steel sheet] Steel Sheet] In particular, in the electrolytic chromate-treated steel sheet in which protrusions were formed at high density on the metal chromium layer on one side of the steel sheet, a phenomenon in which the reproducibility of weldability was poor was confirmed. It is considered that this is because the optimum welding conditions fluctuate due to fluctuations in contact resistance due to the distribution and density of metal chromium projections during welding.

Furthermore, various electrolytic chromate-treated steel sheets having different forms of metal chromium protrusions in the metal chromium layer were manufactured, and as a result of examining the mechanism of destruction of the chromium oxide layer caused by the metal chromium protrusions as described above, the metal was examined. The degree of destruction of the chromium oxide layer due to the chromium projections largely depends on the size and formation density (number per unit area) of the projections. Large chromium metal protrusions, particularly those formed by metal chromium protrusions having a maximum diameter of 10 nm or more when observed from above, are generated by chromium oxidation. It was found that the material layer was easily broken. As a result of further study, if the number of metal chromium protrusions having a maximum diameter (maximum width) of 10 nm or more when observed from above is less than 6 × 10 ¹² / m ² , An excessive decrease in the contact resistance at the plate / plate interface due to the destruction of the chromium oxide layer is prevented, a material steel plate having a surface roughness not less than the above-mentioned predetermined level is used, and the adhesion amount of the chromium oxide layer is made to a predetermined level. It has been found that the contact resistance between the plate / plate interface and the electrode / plate interface can be reduced in a well-balanced manner, in combination with the effect of the reduction described below.

FIG. 1 shows a steel sheet having a surface roughness Ra of not less than 0.15 μm and a number of projections of more than 25 μinch per inch of not less than 10 PPI as a material steel sheet. The formation amount of chromium oxide layer and the plate /
It shows the relationship between the plate interface and the contact resistance at the electrode / plate interface. According to the figure, the length of the maximum diameter portion is 10 nm.
The formation density of the above metallic chromium projections is 6 × 10 ¹² / m
^Two or more electrolytic chromate-treated steel sheets ("Electrochromate-treated steel sheet with protrusion density of 50 × 10 ¹² / m ² " and "Electro-chromate treated steel sheet with protrusion density of 10 × 10 ¹² / m ² ") Then, the contact resistance at the plate / plate interface (in the figure,
The contact resistance indicated by A and B) is much lower than the contact resistance at the electrode / plate interface (contact resistance indicated by D and E in the figure). On the other hand, the length of the maximum diameter portion is 10 nm.
The formation density of the above metallic chromium projections is 6 × 10 ¹² / m
Electrolytic chromate-treated steel sheet of less than ² (protrusion formation density is 1
× 10 ¹¹ pieces / m ² electrolytic chromate-treated steel sheet), the contact resistance at the plate / plate interface (contact resistance indicated by C in the figure) is the contact resistance at the electrode / plate interface (contact resistance indicated by F in the figure). It turns out that it is almost equivalent to.

According to FIG. 1, the length of the maximum diameter portion is 1
The formation density of metallic chromium protrusions of 0 nm or more is 6 × 10 ¹²
In the electrolytic chromate treated steel sheet of less than pieces / m ^2, looking at the relationship between the contact resistance and the adhesion amount of the chromium oxide layer, plate /
In order to sufficiently reduce both the contact resistance at the plate interface and the contact resistance at the electrode / plate interface, the adhesion amount of the chromium oxide layer should be 9%.
It turns out that it is necessary to be less than mg / m ² .

FIG. 2 shows a steel sheet having a surface roughness Ra of 0.15 μm or more and a number of projections exceeding 25 μinch per inch of 10 PPI or more as a material steel sheet, and having a chromium oxide layer adhesion amount of 4 P. The formation density of metal chromium protrusions having a maximum diameter of 10 nm or more formed in the metal chromium layer and the contact resistance at the plate / plate interface and the electrode / plate interface for an electrolytic chromate-treated steel sheet of ９9 mg / m ^2. The contact resistance at the electrode / plate interface is not affected by the formation density of metal chromium protrusions, as described above, but the contact resistance at the plate / plate interface is It is greatly affected by the formation density of projections. According to FIG.
In order to reduce the contact resistance between the plate interface and the electrode / plate interface in a well-balanced manner, the formation density of metal chromium protrusions having a maximum diameter of 10 nm or more in length is less than 6 × 10 ¹² / m ² , and preferably 4 × Less than 10 ¹² / m ² , particularly preferably 3 ×
It turns out that it is necessary to be less than 10 ¹² pieces / m ² .

Regarding the adhesion amount of the metal chromium layer, if the adhesion amount per one side exceeds 150 mg / m ² , metal chromium, which is a high melting point metal, covers the entire surface, thereby deteriorating the pressure contact property during welding and generating dust. And it becomes difficult to ensure non-polishing weldability. Based on the above results, in the present invention, from the viewpoint of ensuring excellent non-polishing weldability, the surface roughness Ra of the material steel sheet was set to 0.1.
The number of protrusions of 5 μm or more and more than 25 μinch per inch is defined as 10 PPI or more, the adhesion amount of the chromium oxide layer is 9 mg / m ² or less per side, and the adhesion amount of the metal chromium layer is 150 mg / m ² per side. Hereinafter, the formation density of metal chromium protrusions having a maximum diameter of 10 nm or more on the surface of the metal chromium layer is defined as less than 6 × 10 ¹² / m ² .
The more preferable density of the metallic chromium projections is less than 4 × 10 ¹² / m ² , particularly preferably less than 3 × 10 ¹² / m ² .

FIG. 3 shows two types of welding conditions (pressing force) for the following material A, which is an electrolytic chromate treated steel sheet satisfying the above conditions of the present invention, and the following material B, which is an electrolytic chromate treated steel sheet which does not satisfy the conditions of the present invention. 63kg, welding speed 1
The ACR at the time of high-speed welding at 7 m / min, a pressure of 53 kg, and a welding speed of 21 m / min) is shown. In the following materials A and B, the projection density of the metal chromium layer is defined as the length of the largest diameter portion existing on the surface of the metal chromium layer being 1
It refers to the formation density of metal chromium protrusions of 0 nm or more. According to the figure, it can be seen that the material A is clearly superior in weldability to the material B.

Material A: Projection density of metal chromium layers on both sides: less than 1 × 10 ¹¹ / m ² Adhesion amount of metal chromium layer: 100 mg / m ² Adhesion amount of chromium oxide layer: 5 mg / m ² plate / plate interface Contact resistance of the electrode / plate interface: 250 μΩ Material B Projection density of the metal chromium layer on the outer surface of the can: 1 × 10 ¹¹ / m
⁽²⁾ Projection density of the metal chromium layer on the inner surface side of the can: 5 × 10 ¹³ / m
² Adhesion amount of metal chromium layer: 100 mg / m ² Adhesion amount of chromium oxide layer: 5 mg / m ² Contact resistance between plate / plate interface: 25 μΩ Electrode / plate interface contact resistance: 250 μΩ

Next, the results of a study on corrosion resistance will be described. 18L cans and pail cans are often used for contents such as chemicals, and particularly for alkaline contents to which tin-based materials cannot be applied. Usually, since these alkaline contents contain various surfactants,
Often the same pH is more corrosive than simple inorganic alkaline contents. The present inventors have investigated the corrosion resistance of electrolytic chromate-treated steel sheets with commercially available alkaline contents (detergents) as typical contents. As a result, when the adhesion amount of the chromium oxide layer exceeds 9 mg / m ² , the adhesion amount of the metal chromium layer is 60 to 200 mg / m ^2.
There was no remarkable change in the corrosion resistance even when changed in the range of ² .
On the other hand, the adhesion amount of the chromium oxide layer was 9 mg / m ^2.
In the following cases, the adhesion amount of the metal chromium layer is 90 mg / m ²
Below, the corrosion resistance was inferior, and thus it was found that the adhesion amount of the metal chromium layer had to be more than 90 mg / m ^{2 in} order to obtain the desired corrosion resistance.

On the other hand, there are acidic contents in addition to alkaline contents in the contents of the can, and when such contents are intended, they are often used by applying an inner surface coating. A relatively smooth form (that is, a metal chromium layer in which the maximum density of the chromium metal protrusions present on the surface of the metal chromium layer is 10 nm or more and the density of protrusions of metal chromium is less than 6 × 10 ¹² / m ² ) In the case of (Form), it was found that when the adhesion amount of the chromium oxide layer was less than 3 mg / m ² , the paint adhesion was deteriorated and corrosion under the coating film was liable to occur.

Further, when the surface roughness of the material steel sheet is increased, the gap between the electrolytic chromate-treated film and the organic resin film thereon is increased, and this gap serves as a starting point of corrosion, deteriorating the corrosion resistance. Specifically, it was found that when the surface roughness Ra of the material steel plate exceeded 0.50 μm and the number of projections exceeding 25 μinch per inch exceeded 120 PPI, the corrosion resistance under the organic resin film deteriorated. Based on the above results, in the present invention, from the viewpoint of securing excellent corrosion resistance, the surface roughness Ra of the material steel plate was set to 0.50.
μm or less, the number of protrusions exceeding 25 μinch per inch is defined as 120 PPI or less, the amount of chromium oxide layer attached is 3 mg / m ² or more per side, and the amount of metal chromium layer attached is 90 mg / m ² per side. The above is defined.

Next, a method for producing the electrolytic chromate-treated steel sheet of the present invention will be described. The electrolytic chromate-treated steel sheet of the present invention has a surface roughness Ra of 0.15 to 0.50 μm and a number of projections exceeding 25 μinch per inch of 10 to 10.
A steel sheet in the range of 120 PPI is used as a material steel sheet, and the material steel sheet is degreased and pickled, and then subjected to electrolytic chromate treatment (cathode electrolytic treatment) in a chromic acid bath.

The deposition efficiency of metallic chromium generated during electrolytic chromate treatment increases with an increase in the current density of the cathodic electrolytic treatment, and the degree of the increase decreases at a current density of 70 ASD (A / dm ² ) or more. On the other hand, the adhesion amount of the chromium oxide layer hardly depends on the current density. Therefore, the chromium oxide layer is subjected to electrolytic treatment at a high current density of 70 ASD or more, whereby the amount of adhesion per unit amount of electricity is suppressed, and the amount of anions contained in the film is also reduced. Therefore, in the manufacturing method of the present invention, the cathodic electrolytic treatment is performed at a high current density of 70 ASD or more. In addition, in the electrolytic chromate treatment at such a high current density, when the number of passes is four or more, the thickness of the chromium oxide layer becomes uneven, and the metal chromium layer also grows in a protruding manner, which is not preferable. . Therefore, in the present invention, the cathodic electrolysis is performed in one to three passes.

[0029] The length of the maximum diameter portion is necessary to suppress the generation of projections of more metal chromium 10nm in metallic chromium layer in the present invention as mentioned earlier, such a metal
In order to suppress the formation of chromium protrusions, the chromium oxide layer, which is the starting point of the formation of metal chromium protrusions, in the non-energized section of the cathodic electrolysis treatment (the part where the steel sheet in the chromic acid bath does not face the electrode) is used. It is necessary to suppress heterogeneous dissolution. As a result of the investigation, in the chromic acid bath in the cathodic electrolysis treatment, the cathodic electrolysis current was passed at a current density of ³ ASD (A / dm ² ) or more even in a portion where the steel sheet did not face the electrode, thereby forming a chromium oxide layer. It has been found that the dissolution of can be effectively suppressed.

On the other hand, in addition to the form of the metal chromium layer and the adhesion amount of the chromium oxide layer, the quality of the chromium oxide layer also affects the contact resistance. The non-uniform quality of the oxide layer causes a variation in the contact resistance, and the content of anions and the like leads to an overall increase in the contact resistance. The generation amount of chromium oxide becomes remarkable when the current density is 7 ASD or more, and the chromium oxide generated at such a low current density is uneven and adversely affects weldability. Therefore, in the cathodic electrolysis, the current density of the cathodic electrolysis current applied to the portion where the steel sheet in the chromic acid bath does not face the electrode needs to be 7 ASD (A / dm ² ) or less.

Therefore, according to the production method of the present invention, in the above-described 1-3 pass cathodic electrolysis treatment, the portion where the steel sheet in the chromic acid bath does not face the electrode (usually the contact portion of the steel sheet in each plating tank with the sink roll) And its vicinity), the cathodic electrolysis current is always supplied at a current density of 3 to 7 ASD. As described above, there is no particular limitation on the method of applying the cathodic electrolysis current at a current density of 3 to 7 ASD in a portion where the steel sheet in the chromic acid bath does not face the electrode. By providing an auxiliary electrode in the vicinity or having a structure in which the sink roll and the electrode are close to each other, a desired cathodic electrolysis current can be supplied.

In the chromic acid bath in which electrolytic chromate treatment is performed, when the anion species present in the bath is only F ions, the chromium oxide layer dissolves remarkably, and chromium metal projections are formed on the surface of the chromium metal layer. Cheap. In order to prevent this, the chromic acid bath needs to contain sulfate ions at a concentration of 1/500 or more of the chromic acid concentration. On the other hand, if the sulfate ion concentration in the chromate film is too high, the corrosion resistance of the chromium oxide layer deteriorates, so the sulfate ion concentration in the chromic acid bath needs to be 1/50 or less of the chromic acid concentration.
The electrolytic chromate-treated steel sheet manufactured under the above conditions has a chromium oxide layer of 9 mg / m ² or less, has little coloration due to anions, and has few projections of metal chromium that absorbs a short wavelength of light. Show the appearance.

[0033]

EXAMPLE A cold-rolled steel sheet having a thickness of 0.32 mm was electrolytically degreased and pickled by a usual method, and was prepared using the following plating baths (chromic acid bath) compositions A to D (both at a bath temperature of 45 ° C.). Electrolytic chromate treatment was performed under the conditions shown in Example 1 to produce an electrolytic chromate treated steel sheet having a metal chromium layer and a chromium oxide layer shown in Table 1 on both surfaces of the steel sheet. The switching of the plating current value was performed by controlling the rectifier by using a function generator (HB105 manufactured by Doo Denko).

Plating bath composition A CrO ₃ : 160 g / L F ion: 2.7 g / L SO ₄ ion: 0.2 g / L Plating bath composition B CrO ₃ : 130 g / L SO ₄ ion: 0.8 g / L L Plating bath composition C CrO ₃ : 110 g / L F ion: 2.7 g / L Plating bath composition D CrO ₃ : 130 g / L SO ₄ ion: 2.0 g / L

With respect to the obtained electrolytic chromate-treated steel sheet, the contact resistance and the morphology of the metal chromium layer (density of formation of metal chromium projections on the metal chromium layer) were measured, and the alkali content resistance, corrosion resistance under the coating film, Weldability, acid-resistant physical properties and color tone were examined. The measurement conditions, test conditions, and performance evaluation methods are shown below. (1) Contact resistance After baking the electrolytic chromate treated steel sheet at 210 ° C for 10 minutes,
With the front and back surfaces together, it is sandwiched between copper chips having a curved surface of 200R, and a current of 1A is applied at a pressure of 100 kg.
The contact resistance value after 3 seconds was determined for the electrode / plate interface and the plate / plate interface.

(2) Morphology of Metal Chromium Layer (Protrusion Density of Metal Chromium) Observation of the electrolytic chromate-treated steel sheet with a transmission electron microscope shows the number of metal chromium protrusions having a maximum diameter of 10 nm or more in the metal chromium layer (forming density). Was measured. Board /
6 × 10 ¹² contacts /
m ² or more was evaluated as “×”, and less than 6 × 10 ¹² particles / m ^{2 was} evaluated as “○”. (3) Alkali content resistance An electrolytic chromate-treated steel sheet was immersed in a nonionic alkaline surfactant (trade name: New Cleaner) kept at 35 ° C, and after 3 weeks, the presence or absence of discoloration was evaluated as follows. . ○: no discoloration ×: discoloration

(4) Corrosion Resistance Under the Coating Film The electrolytically chromated steel sheet coated with the inner surface was passed through a welding machine for an 18L can body, and after a flange was formed, a test material was collected from the can body. This was immersed in a strong alkaline surfactant (trade name: Rypon) kept at 35 ° C.
After the elapse of a week, the presence or absence of rust was evaluated as follows. ○: No rusting ×: Rusting (5) Weldability Using an 18L can body welding machine (VWS, manufactured by Fuji Kogyo Co., Ltd.), electrolytic chromate-treated steel sheet was used at a wire speed of 21 m.
The ACR when welding was performed at a rate of / min was evaluated. Perform continuous canning at three different canning occasions. If the ACR of 5 or more points can always be obtained in the tap range, select "○". If the ACR is always less than 5 points, select "X". "△"

(6) Acid-Resistant Content The electrolytic chromate-treated steel sheet was coated with an epoxyphenol-based paint and baked at 210 ° C. for 10 minutes to prepare a test material having a coating thickness of about 4 μm. The test material was cross-cut, immersed in a 1.5% citric acid + 1.5% salt solution at 38 ° C. for 4 weeks, and the corrosion state of the cut portion was evaluated as follows. ○: No corrosion ×: Corrosion (7) Color Tone 10n between 480nm and 700nm by colorimeter
The reflectance-wavelength gradient was determined at m intervals, and the obtained average value was further corrected to a reflectance difference per 1000 nm to determine a K value (% / 1000 nm). This K value is -20
In the following cases, it will be metallic and good color tone,
The case where the K value was −20 or less was “○”, and the case where the K value was more than −20 was “×”.

Table 2 shows the contact resistance values of the test materials at the plate / plate interface and the electrode / plate interface and the results of the characteristic tests. Comparative Example 1
Since the current density of the cathodic electrolysis treatment (the current density at the part where the electrode and the steel plate face each other) is less than 70 ASD, the formation density of the metal chromium protrusions having a maximum diameter of 10 nm or more formed on the surface of the metal chromium layer is reduced. 6 × 10 ¹² pieces / m ² or more, and therefore, the weldability and the color tone are inferior. In Comparative Example 2, since the current density in a portion where the electrode and the steel sheet did not face each other in the cathodic electrolysis treatment was less than 3 ASD, the formation density of metal chromium protrusions having a maximum diameter portion formed on the surface of the metal chromium layer of 10 nm or more was 10 nm or more. Is 6 × 10 ¹² / m ² or more, and therefore, the weldability and the color tone are inferior.

In Comparative Example 3, the number of electrolytic passes in the cathodic electrolytic treatment was three.
Since the number of passes exceeds the pass, the formation density of metal chromium protrusions having a maximum diameter of 10 nm or more formed on the surface of the metal chromium layer is 6 × 10 ¹² / m ² or more, which results in poor weldability and color tone. I have. Further, since the adhesion state of the chromium oxide layer becomes non-uniform and the amount of adhesion is less than 3 mg / m ² , the acid content properties and the under-coating corrosion resistance are also inferior. In Comparative Example 4, the adhesion amount of the metal chromium layer was 90 mg / m ^2.
, The alkali content resistance and the undercoat corrosion resistance are inferior. In Comparative Example 5, since the current density in the portion where the electrode and the steel sheet did not face each other exceeded 7 ASD, the chromium oxide layer became non-uniform and the amount of adhesion was 9 mg.
/ M ² and the amount of deposited metal chromium layer is 150 mg
/ M ^2, which is inferior in weldability.

In Comparative Example 6, the material steel sheet had a surface roughness Ra of 0.
Since the number of projections less than 15 μm and more than 25 μinch per inch is less than 10 PPI, weldability is poor. In Comparative Example 7, the surface roughness Ra of the material steel plate was 0.50.
Since the number of protrusions exceeding μm and exceeding 25 μinch per inch is more than 120 PPI, the acid content resistance, alkali content resistance and under-coating corrosion resistance are inferior.
Comparative Example 8 is inferior in weldability because the current density in the part where the electrode and the steel sheet do not face each other exceeds 7 ASD.
Comparative Example 9 is inferior in weldability because the amount of the deposited metal chromium layer exceeds 150 mg / m ² .

In Comparative Example 10, since the electrolytic chromate treatment was performed in a chromic acid bath containing no sulfate ion, the density of the projections of the metal chromium having a maximum diameter of 10 nm or more formed on the surface of the metal chromium layer was reduced. 6 × 10 ¹² pieces / m ² or more, and therefore, the weldability and the color tone are inferior. Comparative Example 1
No. 1 is obtained by performing an electrolytic chromate treatment in a chromic acid bath containing an excessive amount of sulfate ions, and thus is inferior in acid content resistance, alkali content resistance, and under-film corrosion resistance.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above, the electrolytic chromate-treated steel sheet of the present invention has a good surface tone, bare corrosion resistance (alkali content resistance) in an alkaline solution containing a surfactant, and acid resistance content properties. And it has excellent corrosion resistance under the coating film and has excellent weldability that enables non-polishing welding of 15 m / min or more. Further, according to the method for manufacturing an electrolytic chromate-treated steel sheet of the present invention, an electrolytic chromate-treated steel sheet having the above-described excellent performance can be efficiently and stably manufactured.

[Brief description of the drawings]

FIG. 1 shows a steel plate having a surface roughness Ra of not less than 0.15 μm and a number of protrusions of more than 25 μinch per inch of not less than 10 PPI as a material steel plate. Graph showing the relationship between the adhesion amount of the chromium oxide layer and the contact resistance of the plate / plate interface and the electrode / plate interface during welding for electrolytic chromate steel sheets having different densities).

FIG. 2 shows a steel sheet having a surface roughness Ra of not less than 0.15 μm and a number of projections exceeding 25 μinch per inch of not less than 10 PPI as a material steel sheet, and an adhesion amount of a chromium oxide layer of 4 to 9 mg / mm. electrolytic chromate treated steel sheet of m ^2, the length of the maximum diameter portion formed on the metallic chromium layer 10
The formation density of metallic chromium protrusions of not less than
Graph showing the relationship between the plate interface and the contact resistance at the electrode / plate interface

FIG. 3 shows two types of welding conditions (a pressure of 63 kg and a welding speed of 1) for an electrolytic chromate treated steel sheet satisfying the conditions of the present invention and an electrolytic chromate treated steel sheet not satisfying the conditions of the present invention.
Graph showing ACR when high-speed welding is performed at 7 m / min, pressure 53 kg, welding speed 21 m / min).

Continuation of the front page (72) Inventor Naoyuki Oba 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-8-209303 (JP, A) JP-A-5-287759 ( JP, A) JP-A-3-177599 (JP, A) JP-A-63-186894 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25D 11/38

Claims (2)

(57) [Claims] 1. A surface roughness Ra of 0.15 to 0.50 μm
A metal chromium layer having an adhesion amount of more than 90 mg / m ² per side and 150 mg / m ² or less on both sides of a steel sheet having a number of projections exceeding 25 μinch per inch in a range of 10 to 120 PPI, An electrolytic chromate-treated steel sheet having a chromium oxide layer having an upper layer having a chromium metal equivalent of 3 to 9 mg / m ² per one surface, and is present on the surface of the metal chromium layer and has a maximum value when observed from above. The number of metal chromium protrusions having a diameter of at least 10 nm is 6 × 10
Electrolytic chromate treated steel sheet for welding excellent in alkali content resistance and weldability characterized by being less than ¹² pieces / m ² . 2. The method for producing an electrolytic chromate-treated steel sheet for welding according to claim 1, wherein the surface roughness Ra is 0.15.
After performing degreasing and pickling on both surfaces of a steel sheet having a thickness of 0.50 μm and a projection exceeding 25 μinch per inch in the range of 10 to 120 PPI, the chromic acid concentration is 1/500 to 1/500. In a chromic acid bath containing 1/50 concentration of sulfate ion, at a current density of 70 ASD or more,
In addition to performing the cathodic electrolysis treatment of the pass, the cathodic electrolysis current is always supplied at a current density of 3 to 7 ASD even in a portion where the steel sheet in the chromic acid bath does not face the electrode in the cathodic electrolysis treatment. A method for producing an electrolytic chromate treated steel sheet for welding having excellent alkali content resistance and weldability.